The iso-paraffin/olefin alkylation process is widely used to manufacture a high octane quality blend component for aviation and motor gasoline which is also valued for its relatively low vapor pressure, low sensitivity and, because of its freedom from aromatic components, its environmental acceptability. The process typically reacts a C3 to C5 olefin with isobutane in the presence of an acidic catalyst to produce the alkylate product.
Hydrofluoric and sulfuric acid alkylation processes have achieved widespread utilization to produce the highly desirable clean burning gasoline component. Although hydrogen fluoride, or hydrofluoric acid (HF) is toxic and corrosive, extensive experience in its use in the refinery have shown that it can be handled safely, provided the hazards are recognized and precautions taken. The HF alkylation process is described in general terms in Modern Petroleum Technology, Hobson et al (Ed), Applied Science Publishers Ltd. 1973, ISBN 085334 487 6. A survey of HF alkylation may be found in Handbook of Petroleum Refining Processes, Meyers, R. A. (Ed.), McGraw-Hill Professional Publishing, 2nd edition (Aug. 1, 1996), ISBN: 0070417962.
In order to improve the safety factors of the HF alkylation process, one option is to operate with a vapor suppressant additive in the alkylation acid. The vapor suppressant additives normally contemplated are those which reduce the volatility of the HF acid. Compounds of this type which have been proposed include organic sulfones, ammonia, amines such as the lower alkylamines (methyl to pentyl), pyridine, alkylpyridines, picoline, melamine, hexmethylenetetramine. A number of different sulfones have been proposed for this purpose but the one generally preferred is sulfolane (tetramethylenesulfone) with 3-methylsulfolane and 2,4-dimethylsulfolane also being suitable. A more detailed description of vapor suppressant additives of this type is given in U.S. Pat. No. 6,114,593 to which reference is made for this description. When a vapor suppressant additive is used the process is often referred to as modified HF alkylation (MHF).
In order to improve the operation of the HF alkylation process—whether with or without the vapor suppressant additive—as well as the economics of the process it is desirable to regenerate the HF alkylation acid by removing the water which accumulates as well as the polymeric by-product formed during the alkylation reactions; this polymer, comprising polymers of differing degrees of conjugation, is frequently referred to as “acid soluble oil” (ASO) since it is miscible with the HF acid phase. Removal of the ASO is necessary to preserve the concentration of the acid at the high level desirable for good alkylation performance while removal of water is required in order to reduce corrosion within the unit as well as to maintain product octane quality; normally, the acid concentration is maintained at 80 to 96 weight percent by the continuous or periodic addition of fresh acid and the regeneration of unit acid; the water content is normally kept in the range of 0.5 to 4.0 percent.
Regeneration of the HF alkylation acid may be accomplished either by internal or external regeneration. In the internal regeneration, a portion of the alkylation acid is injected into the isostripper and stripped off by heated isobutane. A regeneration process of this type is described, for example, in U.S. Pat. No. 4,239,931 (Mikulicz). A separate regeneration tower is used in external regeneration: a portion of the acid stream is stripped with light hydrocarbon in the regeneration tower, typically using a stream of hot isobutane which is subsequently returned to the unit reactor section (reaction settler) or the fractionation section (isostripper overhead) or is condensed and pumped back to the acid inventory in the reactor. Water removal is accomplished by suppressing operations in the external regenerator and dropping an acid/water phase out of the bottom of the column. The use of this stripping technique presents a number of problems: first, if the amount of stripping medium passed into the acid stream is sufficient to vaporize all or almost all of the HF acid, the volume of hydrocarbon is quite large, imposing a relatively large volume requirement on the stripping vessels. In addition, the heavy polymeric material tends to be entrained with the stripping medium and when this polymer enters the downstream equipment, it creates fouling problems, depositing on piping, condensers and the like. Conversely, if attempts are made to limit the volume of stripping medium relative to the acid stream, the stripping will be incomplete and acid will be lost when the bottoms material is dropped out of the regenerator tower. Second, it is difficult to control the regenerator liquid bottoms temperature. For additivated HF units (MHF units) using sulfolane additive, the problems encountered in the removal of the water are compounded by the fact that the distillation properties of sulfolane are similar to the heavy polymer so that the additive will be removed from the unit with the bottoms and so lost from the unit together with excessive amounts of acid.
An external regeneration method is described in U.S. Pat. No. 5,547,909 (Carlson). In the method described here, the acid phase from the settler is removed and a portion is routed to a separator column. Acid, free of ASO and water is removed as overhead and is recirculated to the settler. While this technique may be capable of improving on the conventional external separation by attempting to get closer to an azeotropic mixture of water, acid and polymer in the bottom of the tower, (acid content of the bottoms stream about 40-50%), it still fails to achieve a satisfactory level of acid recovery and significant losses of acid can be expected.
A regeneration process which is stated to remove polymer and water from the an MHF system is described in U.S. Pat. No. 5,759,937 (Hovis et al). In this process, the alkylation catalyst is subjected to stripping in a regenerator which has a vapor sidedraw of water, HF and stripper medium. This sidedraw is partially condensed resulting in a vapor stream comprising isoparaffin stripping medium and one or more liquid streams which may variously contain water and isoparaffin. In actual operation, however, the partial condensation stated to be critical is impractical since the control required for targeting the desired temperature is not easily accomplished by cooling. Operation in the practical realm, therefore, tends to condense the entire stream and to purge the condensed acid to the waste treatment plant. The composition of the purged stream is normally about 8 parts of acid to one part of water (90% acid), indicating that a substantial loss of acid takes place.
Accordingly, there is a need for an improved technique for regenerating HF alkylation acid by separating it from water and polymer as well as from vapor suppressant additives in MHF units.